Refining processes for semiconductor materials



Nov. 24, 1959 H. F. STERLENG 2,914,397

REFINING PROCESSES FOR SEMICONDUCTOR MATERIALS Filed 001:. 7, 1957 FI.G.2.

cumg/yr sou/x5 Inventor H. F: fiterllnc United States Patent REFINING PROCESSES FOR SEMICONDUCTOR MATERIALS Henley Frank Sterling, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application October 7, 1957, Serial No. 688,610

Claims priority, application Great Britain October 16, 1956 1 Claim. (CI. 75-63) The present invention relates to a process for treating by melting semiconductors and other meltable materials.

The invention comprises an improvement in, or modification of, the invention described and claimed in the specification of co-pending application No. 618,272, filed October 25, 1956, which for convenience will be called the parent specification.

The object of the present invention is to provide some additional melting processes for treating semiconductors and other meltable materials.

The parent specification describes a process in which a rod of silicon is arranged with its axis vertical and is surrounded by a single-turn heating coil arranged eccentrically with respect to the axis of the rod. An annular molten region is thereby produced in the silicon rod, which region surrounds a solid core which is also eccentric to the axis. By suitable mechanism the rod is rotated and at the same time moved vertically so that the solid core follows a helical path in the rod. By this means every portion of the silicon is eventually melted, but there is always some part of the cross-section of the rod which is solid from end to end, and the molten region is supported and prevented from spilling partly by surface tension, and partly by the force due to the electromagnetic field of the high-frequency coil which heats the molten region.

According to the present invention there is provided a process for treating a rod of meltable material which comprises arranging the rod in a vertical position, melting an annular zone surrounding an unmelted portion in the said rod, and causing the molten zone to traverse a straight path parallel to the axis of the rod, whereby all the material of the rod is subjected to melting except an inner substantially cylindrical core portion which is parallel to the axis.

The present invention will be described with reference to the drawing, in which:

Fig. 1 shows a longitudinal sectional view of apparatus for zone refining silicon or other semiconductor according to the invention;

Fig. 2 shows a partly sectional plan view of Fig. 1; and

Fig. 3 shows a modification of Fig. 2.

The apparatus shown in Fig. l is basically similar to that described in the parent specification, and comprises a cylindrical glass or quartz envelope 1 sealed to a metal disc 2. The envelope 1 has an inlet pipe 3 for argon or other inert gas, which fills the envelope, and escapes at the outlet pipe 4 at the upper end of the envelope. Through a cylindrical bush 5 in the plate 2 passes a shaft 6, the upper end of which carries a socket 7 which holds a rod 8 of silicon or other semiconductor. A single-turn concentrator coil 9, a plan view of which is shown in Fig. 2, surrounds the envelope 1. The coil 9 is placed eccentrically with respect to the axis of the rod 8 so that the left-hand side of the coil is nearer the rod than the right-hand side, as shown.

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As shown inFig. 2, the coil 9 consists of a metal plate with a circular hole 10, but the plate is divided by a narrow slot 11 so that it forms almost a complete single turn coil. A high frequency current source 12 is connected to points in the plate on opposite sides of the slot 11 by conductors 13 and 14. The inner edge of the hole 10 is bevelled as indicated at 15 (Fig. 1).

With this arrangement the electromagnetic field of the coil 9 is applied to the rod 8, and an annular portion is melted, but the cross-section of the annulus on the lefthand side at 16 is larger than that on the right-hand side at 17, and the central solid portion 18 is thus. eccentric towards the right-hand side, as seen in Fig. 2. The liquid portion is held in place partly by surface tension and partly by the force due to the electromagnetic field of the coil 9. The bevelling at 15 is provided to shape the field in such manner that the force acting on the molten portion of the rod tends to hold it in place.

The shaft 6 is now slowly moved upwards without rotation by a suitable mechanism (not shown) so that the melted portion of the rod efiectively travels downwards and so traverses a straight path parallel to the axis of the rod. At the end of the process an eccentric, roughly cylindrical portion of the rod, the cross-section of which is indicated by 18 in Fig. 2, has clearly remained solid throughout, and so has not been subjected to zone-refining. Therefore according to the invention, the process is repeated after the shaft 6 has been turned through Then the portion of the rod which previously remained solid is now melted and another portion indicated dotted at 19 in Fig. 2, in the other half of the rod, remains solid. By this means the whole of the rod is ultimately subjected to melting.

Clearly, instead of rotating the shaft 6 through 180, the coil 9 may be shifted to the left so that the righthand side is nearer to the rod 8 than the left-hand side.

It may be necessary to adjust the magnitude of the current supplied to the coil 9, and perhaps, also, the frequency of the current, so that desired amount of silicon is melted, and also so that a sufficient lifting force is applied to the melted silicon to hold it in place.

If desired, according to the invention, the coil 9 may be arranged concentrically with the rod 8, as shown in the plan view of Fig. 3. In that case, a concentric annular zone of the rod 8 is melted, which surrounds a solid portion 21. When the shaft 6 is moved vertically, a coaxial skin of the rod is progressively melted and solidified, there being left a coaxial unmelted cylindrical core, the cross-section of which is shown at 21. This process may be applied with advantage to roughly cylindrical silicon ingots which have been produced by thermal decomposition of silane, for example by the process described in the specification of co-pending application No. 688,452, filed October 7, 1957. Such ingots tend to have a rough and irregular surface, with fine and deep indentations, and the progressive melting and solidifying of the outside skin by the process described with reference to Fig. 3 smooths out the surface, and reduces the effective surface area, and so the liability to contamination by the atmosphere, and by subsequent handling, is also reduced.

In the processes described with reference to the figures of the drawing, the rod 8 may be moved vertically at the rate of about 1 inch per hour, for example.

Although the invention has been described for treating semiconductor materials, it will be understood that it may be applied to other meltable materials.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly example and not as a limitation on the scope of the invention.

What I claim is:

A process for removing impurities from a rod of semiconductor material which comprises arranging the rod in a substantially vertical position, melting an asymmetrical annular zone surrounding an unmelted portion of the rod, the greatest depth of melt extending beyond the central linear axis of said rod, relatively moving said melted zone from a first end to the second end of said rod in a substantially rectilinear line, whereby impurities from the portions of the semiconductor which have been melted are concentrated at said second end, then melting a second asymmetrical annular zone surrounding said rod similar to said first zone but displaced around said 1 rod substantially one hundred eighty degrees, and relatively moving said second annular zone from said first end of said rod to said second end of said rod, whereby impurities remaining in said first mentioned unmelted portion are likewise concentrated at said second end of said rod.

References Cited in the file of this patent UNITED STATES PATENTS 2,125,173 Kinzel July 28, 1938 2,477,411 King July 26, 1939 2,739,088 Pfann Mar. 20, 1956 FOREIGN PATENTS 1,107,076 France Aug. 3, 1955 

1. A PROCESS FOR REMOVING IMPURITIES FROM A ROD OF SEMICONDUCTOR MATERIAL WHICH COMPRISES ARRAGING THE ROD IN A SUBSTANTIALLY VERTICAL POSITION, MELTING AN ASYMMETRICAL ANNULAR ZONE SURROUNDING AN UNMELTED PORTION OF THE ROD, THE GREATEST DEPTH OF MELT EXTENDING BEYOND THE CENTRAL LINEAR AXIS OF SAID ROD, RELATIVELY MOVING SAID MELTED ZONE FROM A FIRST END OF THE SECOND END OF SAID ROD IN A SUBSTANTIALLY RECTILLINEAR LINE, WHEREBY IMPURITIES FROM THE PORTIONS OF THE SEMICONDUCTOR WHICH HAVE BEEN MELTED ARE CONCENTRATED AT SAID SECOND END, THEN MELTING A SECOND SSYMMETERICAL ANNULAR ZONE SURROUNDING SAID ROD SIMILAR TO SAID FIRST ZONE BUT SIPLACED AROUND SAID ROD SUBSTANTIALLY ONE HUNDRED EIGHTY DEGREES, AND RELATIVELY MOVING SAID SECOND ANNULAR ZONE FROM SAID FIRST END OF SAID ROD TO SAID SECOND END OF SAID ROD, WHEREBY IMPURITIES REMINING IN SAID FIRST MENTIONED UNMELTED PORTION ARE LIKEWISE CONCENTRATED AT SAID SECOND END OF SAID ROD. 